Biological control of European red mite,Panonychus ulmi(Koch), and apple rust mite,Aculus schlechtendali(Nalepa), varied in 1991 in 20 prey-predator treatment plots after Single and mixed releases ofMetaseiulus occidentalis(Nesbitt) andTyphlodromus pyriScheuten were made in 1990. Control of spider mites in 1991 was better in mixed-species than in single-species release plots.M. occidentaliswas the dominant phytoseiid in mixed-species release plots in 1990, butT. pyriwas more common in 1991. In fall, eggs ofM. occidentalisdisappeared before eggs ofT. pyridid. In spring, eggs ofT. pyriappeared first. Oviposition byM. occidentalismay have been slowed by cool weather and limited use of pollen as food. With few prey present, competition favoredT. pyri, andM. occidentaliseventually disappeared from mixed-, but not from single-species plots.M. occidentaliswas more affected byZetzellia mali(Ewing) than wasT. pyri, butT. pyrihad more impact on populations of this stigmaeid mite. Competition byZ. maliwith the phytoseiids may cause negative and pOSitiveeffects on biological control. The two phytoseiids have complementary traits that allow them to control pest mites together, but their management may be complicated by interspecific competition.